The present application provides a cap assembly and a bottle assembly. The cap assembly is configured to lock a bottle. The bottle includes a bottle head having at least one lock accommodating portion and comprises a bottle body. The cap assembly includes a cap, an elastic arm and at least one lock portion. The cap defines an annular receiving portion having an opening facing downward, and the bottle head can enter the receiving portion through the opening. The elastic arm is annular and is arranged in the receiving portion. The at least one lock portion is arranged on the cap and is configured to be capable of cooperating with the at least one lock accommodating portion of the bottle head, so as to lock the cap assembly to the bottle head. When the cap assembly is locked to the bottle head, the elastic arm abuts against a top wall of the receiving portion and a top wall of the bottle head, to apply forces in opposite directions to the cap and the bottle head, so as to retain the at least one lock portion in the at least one lock accommodating portion. The cap assembly of the present application has a relatively small height, and during unlocking of the bottle assembly of the present application, gas in a bottle body cavity can be discharged from the bottle through an exhaust passage.